1. Field of the Invention
This invention relates to color inkjet recording films, and, more particularly to color-receptive films coated with substantially colorless copolymers of vinyl pyrrolidone (VP) and vinyl imidazole VI), characterized in that the color images obtained exhibit excellent light fastness.
2. Description of the Prior Art
The advent of color inkjet printing has been instrumental in fueling the print-on-demand revolution and has also created a number of challenges. Often, the surface of the desired media does not possess the necessary properties for accepting the ink-jet ink. This results in long dry times and/or a poor ink-jet image. It has long been recognized that a surface treatment or media coating plays a critical role in the final print quality. Numerous media coatings are known in the art. They may contain any number of components and often consist of more than one layer. These ink-receptive coatings generally contain at least one hydrophilic polymer; often poly(vinylpyrrolidone) (PVP). PVP brings many benefits to properly formulated media coatings including rapid ink dry time, excellent print quality, highly resolved circular dots, and high, uniform optical density. Furthermore, copolymers of vinylpyrrolidone (VP) along with other suitable comonomers, such as dimethylaminoethyl methacrylamide (DMAPMA), acrylic acid, or vinyl acetate, have been used separately or in conjunction with PVP, to further optimize performance. However, it is desired also to provide long-term, excellent water-resistant qualities for such films.
Commercial VP/VI copolymers e.g. BASF""s LUVITEC(copyright) (90:10 to 30:70 wt/wt) are yellow in color. When coated onto a white polyester film for digital printing inkjet (DPI) applications, these copolymers leave a yellow cast on the printed film.
Accordingly, it is an object of this invention to provide an improved VP/VI copolymer for such DPI applications so that the printed films exhibit excellent light fastness.
These and other objects of the invention will be made apparent from the following description.
What is described herein is a method of digital inkjet printing in which a substrate is coated with a substantially colorless, 90-10 to 30-70, wt/wt, copolymer of vinyl pyrrolidone (VP) and vinyl imidazole (VI) uncrosslinked or crosslinked, followed by inkjet printing dye images on the thus-coated substrate, characterized in that the dye images obtained exhibit excellent light fastness.
Preferably the copolymer has an APHA color rating of  less than 125, most preferably  less than 50, which indicates that it is substantially colorless; and the copolymer has a VP/VI monomer weight ratio of about 50/50; and still more preferably, the light fastness of dye images obtained on the coated film extends for at least one year.
Commercial VI monomer has an APHA color rating of 3850; accordingly, copolymers of VP/VI (50/50 wt/wt) in water have an APHA color rating of only 231.6. DPI applications using such polymers for coating substrates require an APHA color of the coating polymers of  less than 125 so that the dye image will be unaffected by the inherent color of the coating itself. Most particularly, high APHA color rating copolymers are observed to leave a color cast on the printed film.